To a scalemic mixture enriched in (1S,3R,4S)-3-ethyl-4-(6-tosyl-6H-pyrrolo[2,3-e][1,2,4]triazolo[4,3-a]pyrazin-1-yl)cyclopentanol (6.11 g, 14.4 mmol) in pyridine (100 mL) was added DMAP (1.93 g, 15.8 mmol) and 4-nitrophenyl carbonochloridate (4.34 g, 21.5 mmol). The resulting mixture was stirred at ambient temperature for about 3.5 h and heated at about 33° C. for about 1 h. The solid was filtered off and washed with EtOAc (30 mL). The filtrated was concd under reduced pressure. The residue was purified using silica gel chromatography eluting with 0-30% EtOAc in heptane to give a scalemic mixture enriched in (1S,3R,4S)-3-ethyl-4-(6-tosyl-6H-pyrrolo[2,3-e][1,2,4]triazolo[4,3-a]pyrazin-1-yl)cyclopentyl 4-nitrophenyl carbonate (6.63, 78%). LC/MS (Table 1, Method b) Rt=2.65 min; MS m/z: 591 (M+H)+.
